1. Field of the Invention
The present invention relates to microwave components, and particularly to superconduction microwave components such as microwave resonators and microwave delay lines, which are passive components for handling electromagnetic waves having a very short wavelength such as microwaves and millimetric waves, and which have conductor layers formed of oxide superconductor materials. More specifically, the present invention relates to a novel method for fabricating a substrate which can be used for fabricating microwave components having conductor layers formed of oxide superconductor thin films.
2. Description of Related Art
Microwaves and millimetric waves are characterized by a straight-going property of radio waves, reflection by a conduction plate, diffraction due to obstacles, interference between radio waves, optical behavior when passing through a boundary between different mediums, and others. In addition, some physical phenomena which was too small in effect and therefore could not be utilized in practice will remarkably appear in the microwaves and millimetric waves. For example, there are now actually used an isolator and a circulator utilizing a gyro magnetic effect of a ferrite, and medical instruments such as plasma diagnosis instrument utilizing interference between a gas plasma and a microwave. Furthermore, since the frequency of the microwaves and millimetric waves is extremely high, the microwaves and millimetric waves have been used as a signal transmission means of a high speed and a high density.
In the case of propagating an electromagnetic wave in frequency bands which are called the microwave and the millimetric wave, a twin-lead type feeder used in a relative low frequency band has an extremely large transmission loss. In addition, if an inter-conductor distance approaches a wavelength, a slight bend of the transmission line and a slight mismatch in connection portion cause reflection and radiation, and is easily influenced from adjacent objects. Thus, a tubular waveguide having a sectional size comparable to the wavelength has been actually used. The waveguide and a circuit constituted of the waveguide constitute a three-dimensional circuit, which is larger than components used in ordinary electric and electronic circuits. Therefore, application of the microwave circuit has been limited to special fields.
However, miniaturized devices composed of semiconductor have been developed as an active element operating in a microwave band. In addition, with advancement of integrated circuit technology, a so-called microstrip line having a extremely small inter-conductor distance has been used.
In 1986, Bednorz and Muller discovered (La, Ba).sub.2 CuO.sub.4 showing a superconduction state at a temperature of 30 K. In 1987, Chu discovered YBa.sub.2 Cu.sub.3 O.sub.y having a superconduction critical temperature on the order of 90 K, and in 1988, Maeda discovered a so-call bismuth (Bi) type compound oxide superconductor material having a superconduction critical temperature exceeding 100 K. These compound oxide superconductor materials can obtain a superconduction condition with cooling using an inexpensive liquid nitrogen. As a result, possibility of actual application of the superconduction technology has become discussed and studied.
Phenomenon inherent to the superconduction can be advantageously utilized in various applications, and the microwave component is no exceptions. In general, the microstrip line has an attenuation coefficient that is attributable to a resistance component of the conductor. This attenuation coefficient attributable to the resistance component increases in proportion to a root of a frequency. On the other hand, the dielectric loss increases in proportion to increase of the frequency. However, the loss in a recent microstrip line is almost attributable to the resistance of the conductor, since the dielectric materials have been improved. Therefore, if the resistance of the conductor in the strip line can be reduced, it is possible to greatly elevate the performance of the microstrip line.
As well known, the microstrip line can be used as a simple signal transmission line. However, if a suitable patterning is applied, the microstrip line can be used as an inductor, a filter, a resonator, a delay line, a directional coupler, and other passive microwave circuit elements that can be used in a hybrid circuit.
The microstrip line is generally constituted of a pair of conductors separated from each other by an dielectric material layer, one of the conductor being grounded.
Here, considering a substrate which can be used for fabricating a microstrip line having conductors formed of oxide superconductor material, it could be estimated that the substrate comprises a plate formed of a dielectric material and a pair of oxide superconduction material thin films deposited on opposite surfaces of the dielectric plate, respectively. If this substrate is given or available, a superconduction microwave component having a predetermined pattern of conductor formed on one surface of a dielectric plate and a grounded conductor formed on the other surface of the dielectric plate, can be easily fabricated by suitably patterning one of the pair of oxide superconduction material thin films.
However, it is very difficult to form the above mentioned substrate for the microwave component, by a process of depositing an oxide superconduction thin film on one surface of a dielectric plate, and thereafter depositing another oxide, superconduction thin film on the other surface of the dielectric plate.
The reason for this is as follows: First, the oxide superconduction thin film can be obtained only when a film of oxide superconductor material is deposited on a specific; substrate under a specific deposition condition. Secondly, since oxygen contained in the oxide superconductor material is unstable, if the deposited oxide superconductor material is heated, the content of the oxygen will change. Therefore, in the process of sequentially depositing an oxide superconduction thin film on one or first surface of a dielectric plate, and thereafter on the other or second surface of the dielectric plate, the superconductor characteristics of the oxide superconduction thin film deposited on the first surface of a dielectric plate is deteriorated when the oxide superconduction thin film is deposited on the second surface of a dielectric plate. Because of this, it has been considered to difficult to prepare a substrate for microwave component having superconduction thin film of good characteristics uniformly formed on opposite surfaces of the substrate.